My employer was a long time AT&T customer for regular cell phones and in the beginning for Blackberries. My department started the move to VZW because of the coverage and now all 200 devices are on VZW.

And in my travels, I have found VZW to be superior in more rural areas everywhere and if you look at their map, you'll see EVDO Rev A almost nationwide. AT&T isn't that fast everywhere. (Of course unless you use a Storm, you aren't getting Rev A anyway)